Use of silica as agent for controlling the degradation of...

Drug – bio-affecting and body treating compositions – Inorganic active ingredient containing – Carbonate

Reexamination Certificate

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C424S401000, C424S049000, C514S970000

Reexamination Certificate

active

06284288

ABSTRACT:

The subject-matter of the present invention is the use of silica as agent for controlling the thermal degradation of alkali metal, alkaline earth metal or ammonium bicarbonate and the use of the silica/bicarbonate mixtures thus obtained in applications in cosmetics and detergency.
Bicarbonates with a very fine particle size, that is to say less than or equal to 100 &mgr;m, are well known constituents of cosmetic and detergent formulations, in particular because of their abrasive and pH-control properties.
However, they exhibit a disadvantage which relates to their thermal degradation temperature. Thermal degradation is understood to mean particularly the temperature at which carbon dioxide is given off. For example, in the preparation of toothpaste formulations, the usual temperature of mixing is of the order of 50° C., in order to avoid any degradation of the bicarbonate which they comprise. Now, it would be advantageous if this temperature could be high, so as to improve the productivity of the process for the formulation of the toothpaste.
In addition, the bicarbonate exhibits another disadvantage related to its very high ability to cake, an ability which becomes increasingly marked as the particle size becomes finer. This in particular presents problems during the use of the said bicarbonate after a more or less lengthy storage. It is therefore necessary to be able to have available a bicarbonate exhibiting a better stability on storage.
These problems and others are solved by the present invention, which therefore consists of the use of silica as a mixture with alkali metal, alkaline earth metal or ammonium bicarbonate with the aim of thermally stabilizing the said bicarbonate, the latter exhibiting a mean particle size of less than or equal to 100 &mgr;m.
It has been found, entirely unexpectedly, that bicarbonate with a very fine particle size could be stabilized when it existed as a mixture with silica, not only during storage but also during its use. This is because the bicarbonate thus treated with the additive exhibits an improved flowability with respect to the bicarbonate alone, as well as a decreased ability to cake. In addition, the temperature range in which the bicarbonate degrades, that is to say the temperature range in which departure of carbon dioxide is observed, is shifted towards higher temperatures and their range is broadened.
Finally, the mixture according to the invention exhibits the advantage of being used directly by the user. This is because the composition of the mixture according to the invention is compatible with applications in cosmetics and detergency in particular, such as, for example, the formulation of toothpaste. This allows the applicator to do without an additional stage of metering bicarbonate and silica, while having available a mixture which does not exhibit the disadvantages of the bicarbonate alone.
However, other characteristics and advantages of the present invention will become more clearly apparent on reading the description and examples which will follow.
The first object of the invention is therefore to thermally stabilize the bicarbonate of fine particle size.
The bicarbonate can be an alkali metal, alkaline earth metal or ammonium bicarbonate. More particularly, the stabilized bicarbonate is an alkali metal or alkaline earth metal bicarbonate. The bicarbonate is preferably a sodium bicarbonate.
The mean size of the bicarbonate particles is less than or equal to 100 &mgr;m. According to a specific embodiment of the invention, the mean size of the bicarbonate particles is between 10 and 90 &mgr;m. More particularly, the mean size of the bicarbonate particles is between 20 and 80 &mgr;m and preferably between 20 and 60 &mgr;m. The particle size is measured by using an AFNOR sieve or by laser diffraction using a Sympatec particle sizer.
The bicarbonate is generally obtained by employing a process comprising essentially two parts. Thus, in a first stage, alkali metal, alkaline earth metal or ammonium carbonate is prepared, according to the Solvay process for example. In a second part, an aqueous solution is prepared by dissolving the carbonate thus obtained and then carbon dioxide is injected in order to obtain the bicarbonate.
Commercially available bicarbonates are suitable for the present invention. However, the present invention is particularly advantageous in stabilizing bicarbonates with an extra-fine particle size.
The silica will now be described.
According to the present invention, a precipitated silica is more particularly employed.
Precipitated silica is understood to mean here a silica obtained by precipitation from the reaction of an alkaline silicate with an acid, generally an inorganic acid, at an appropriate pH of the precipitation medium. In particular, the pH employed is basic, neutral or only slightly acidic. Any method can be used for the preparation of silica; it can, for example, consist of the addition of acid to a silicate vessel heel or alternatively the complete or partial simultaneous addition of acid or a silicate to a water or silicate solution vessel heel, for example.
Use may be made, in the present invention, of silicas with abrasive or alternatively thickening property. It should be noted that the property of the silica is conventionally dependent on the conditions under which the precipitation is carried out.
According to a preferred embodiment of the invention, the silica employed has an abrasive property.
Silicas which have subsequently been subjected to a calcination stage at high temperature, for example greater than or equal to 450° C., can also be employed.
However, it is preferable to use a silica which has not been subjected to such a treatment.
According to another characteristic of the silica used as stabilizer for the bicarbonate, it exhibits a mean size of the particles of between 8 and 30 &mgr;m. More particularly, the mean size of the particles is between 8 and 25 &mgr;m and preferably between 10 and 20 &mgr;m. The mean size of the particles is generally determined by laser diffraction on a Sympatec particle sizer.
Likewise, the precipitated silica used in the present invention preferably have a BET specific surface of between 140 and 300 m
2
/g, in particular between 210 and 250 m
2
/g. The BET specific surface is determined according to the Brunauer-Emmet-Teller method described in “The Journal of the American Chemical Society”, vol. 60, page 309, February 1938, which corresponds to ISO Standard 5764/1 (Appendix I).
The silica employed in the present invention preferably exhibits a DOP oil uptake of between 250 and 370 ml/100 g, in particular between 300 and 350 ml/100 g. The DOP oil uptake is determined according to ISO Standard 787/5 by employing dioctyl phthalate.
A silica with a total pore volume of greater than 2.10 cm
3
/g is advantageously employed. The total pore volume is generally less than 5 cm
3
/g. The pore volume can be measured by mercury porosimetry.
The silica employed in the present invention usually exhibits a pH of between 6.8 and 7.5. The pH is measured according to ISO Standard 787/9 (pH of a 5% suspension in water).
Tixosil 43 or Tixosil 103, sold by the company Rhône-Poulenc, is generally used as precipitated silica.
According to another characteristic of the invention, the amount of silica employed is more specifically between 0.1 and 15% by weight with respect to the weight of bicarbonate.
More particularly, the amount of silica varies between 1 and 12% by weight. It is preferably between 2 and 10% by weight and, according to an even more specific embodiment, of between 4 and 10% by weight.
The silica and the bicarbonate can be brought into contact in an entirely advantageous way by simple dry mixing of the two products. Mixers of any type are used, insofar as they make it possible to obtain homogeneous distribution of the silica within the bicarbonate. Use is preferably made of a double-ribbon blender.
Thus, in an entirely advantageous way, it is not necessary to use additional additives, such as binding agents, for example.
It should be noted, in a

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